This is very much based on the Craig Anderton design found in the book "Electronic Projects for Musicians". Pretty gong show ring mod ;).

I made some with probes and some without. These are no longer being made.

The pots control effect volume, clean volume and modulation frequency. The Small toggle switch turns on and off a low pass filter. Again the video shows a unit with the probe, but the black one available now does not have the probe, otherwise it is identical.